Medical Device Daily National editor

Not all nanomaterials are created equal, or, we can assume, will function the same way.

Yes, they are all very, very small, but they vary, very greatly in size and the relative differences in sizes – as great as differences in micro and macro materials – may have important effects in combination and contact with other materials, for instance, and rather importantly, living cells.

The need to determine the differentiating characteristics of nanomaterials of different sizes was a key driver for the development of a new, 3-D nanofluidic device by researchers at the National Institute of Standards and Technology (NIST; Gaithersburg, Maryland) and Cornell University.

Based on the development of microfluidic and other nanofluidic devices, the researchers took this concept and added an additional dimension to the traditional flat, 2-D architecture of these devices, according to Samuel Stavis, PhD, a National Research Council Research Associate at NIST.

As described in their paper, published online in the journal Nanotechnology, Vol. 20 (online March 31; in print April 22), these chambers, Lilliputian but significantly more complex than conventional 2-D devices, were created using a photolithographic process, the prototype system pointing the way to future tools for manipulating and measuring different sizes of nanoparticles in solution, Stavis said.

He told Medical Device Daily that future applications of 3-D nanofluidic devices have "high impact" potential and include the separation and measurement of complex nanoparticle mixtures for drug delivery, gene therapy and nanoparticle toxicology; and the isolation and confinement of individual DNA strands for analysis.

Nanofluidic and microfluidic devices – used to perform chip-based laboratory processes – are often fabricated by etching channels into a glass or silicon wafer using a lithographic procedure for manufacturing computer chip circuit patterns. These channels are then bonded with a glass cover.

But the result is a device with simple geometries and only a few depths, limiting their ability to separate mixtures of very small particles or study the nanoscale behavior of biomolecules in detail.

Stavis and Michael Gaitan, another NIST researcher, and Elizabeth Strychalski of Cornell used gray-scale photolithography – a "masking" process using light for microcircuit engraving to create a nanofluidic chamber with an etched-in "staircase" geometry. (Grayscale photolithography uses "shades of gray" to create the 3-D photo-resist sculpting process in three dimensions.)

Each "step" in this geometric architecture gives the device a progressively increasing depth from 10 nanometers (roughly 6,000 times smaller than the width of a human hair) at the top to 620 nanometers (slightly smaller than an average bacterium) at the bottom.

With this staircase architecture, the device is able to manipulate nanoparticles by size, much like the way a coin sorter separates nickels, dimes and quarters.

Stavis called the fabrication process "quite simple" but with "a lot of power in terms of fabricating complex structures for different applications."

As one demonstration of the device's ability to investigate the behavior of individual DNA molecules in confined environments, the researchers used their nanofluidic device to manipulate DNA strands in the system's channels using electrophoresis, a method for employing an electric field to force charged particles through a solution. This should demonstrate the device's precision and agility since separate DNA strands tends to roll up into a ball, making analysis difficult.

"We started out with the question," Stavis said, "of how to separate mixtures of nanoparticles and biomolecules and sort them into groups of individual sizes."

Patent documentation is being prepared for the method, but Stavis said that his primary motivation is in seeing the method used as an essential process for manipulating and measuring nanoscale materials, for instance in establishing safety.

By separating and measuring the nanoscale components of a mixture, he said, "you could do a more precise investigation of how nanoparticles interact with cells for therapeutic applications or toxicology investigations."

He added that "because the integrated circuit manufacturing tools and techniques that we use are well established, our fabrication process is easy to adapt and integrate."

"I've been working in the field for a few years to analyze single molecules," and when he realized the possibility of using grayscale photolithography for this application, he said, "This is a good idea, and someone is going to do it."

In their experiments with the device, the NIST-Cornell researchers tested it with two different solutions: one containing 100-nanometer-diameter polystyrene spheres, the other containing 20-micrometer (millionth of a meter)-length DNA molecules from a virus that infects the common bacterium Escherichia coli.

In each experiment, the solution was injected into the deep end of the chamber and then electrophoretically driven across the device from deeper to shallower levels. Both the spheres and DNA strands were tagged with fluorescent dye so that their movements could be tracked with a microscope.

These results show that the 3-D nanofluidic device successfully excluded rigid nanoparticles based on size and deformed (uncoiled) the flexible DNA strands into distinct shapes at different steps of the staircase.

Currently, the researchers are working to separate and measure mixtures of different-sized nanoparticles and investigate the behavior of DNA captured in a 3-D nanofluidic environment.

In a previous project, the NIST-Cornell researchers used heated air to create nanochannels with curving funnel-shaped entrances in a process they dubbed "nano-glassblowing." Like its new 3-D cousin, the nano-glassblown nanofluidic device facilitates the study of individual DNA strands.

The work described in the recent Nanotechnology paper was supported in part by the National Research Council Research Associateship Program and Cornell's Nanobiotechnology Center, part of the Science and Technology Center Program of the National Science Foundation.

The 3-D nanofluidic devices were fabricated at the Cornell Nanoscale Science and Technology Facility and the Cornell Center for Materials Research, and characterized at the NIST Center for Nanoscale Science and Technology. All experiments were performed at the NIST laboratories.

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